Flutter sensing and control system for a gas turbine engine

ABSTRACT

A gas turbine engine includes a fan section that has a fan. A fan casing surrounds the fan section. A compressor section defines an engine axis. A gear train reduces a rotational speed of the fan relative to a shaft. A low pressure turbine is coupled to the shaft. A nacelle extends along the engine axis and surrounds the fan casing, a bypass ratio of greater than 10. A variable area fan nozzle defines a discharge airflow area. A flutter sensing system includes a controller programmed to move the variable area fan nozzle to vary the discharge airflow area in response to detecting an airfoil flutter condition associated with adjacent airfoils of the fan.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/340,747, filed 30 Dec. 2011, which is a continuation of U.S. patentapplication Ser. No. 11/682,015, which was filed on 5 Mar. 2007 and isincorporated herein by reference.

BACKGROUND

This invention generally relates to a gas turbine engine, and moreparticularly to a flutter sensing system for a gas turbine engine.

Gas turbine engines typically include a compressor section, a combustorsection and a turbine section. Air is pressurized in the compressorsection and is mixed with fuel and burned in the combustor section toadd energy to expand the air and accelerate the airflow into the turbinesection. The hot combustion gases that exit the combustor section flowdownstream through the turbine section, which extracts kinetic energyfrom the expanding gases and converts the energy into shaft horsepowerto drive the compressor section.

In a turbofan gas turbine engine, for example, a fan section is includedupstream of the compressor section. Combustion gases are discharged fromthe gas turbine engine through a core exhaust nozzle and fan air isdischarged through an annular fan exhaust nozzle defined at leastpartially by a nacelle surrounding the core engine. A majority ofpropulsion thrust is provided by the pressurized fan air which isdischarged through the fan exhaust nozzle, while the remaining thrust isprovided from combustion gases discharged through the core exhaustnozzle.

A fan section, the compressor section and the turbine section mayinclude multiple airfoils disposed circumferentially about an enginelongitudinal centerline axis. At certain aircraft operating conditions,these airfoils may be subjected to flutter, or self-inducedoscillations. The flutter conditions are caused by the interactionbetween adjacent airfoils. During flutter, aerodynamic forces couplewith each airfoil's elastic and inertial forces, which may increase thekinetic energy of each airfoil and produce negative damping. Thenegative damping is enhanced where adjacent airfoils vibrate in unison.Disadvantageously, the airfoil oscillations caused by flutter may becomeso severe that fracture or failure of the airfoils is possible.

Methods are known for mitigating the negative effects of flutter. Forexample, many gas turbine engine systems include high pressurecompressors having variable vane rows (i.e., vanes that are rotatableabout a perpendicular axis relative to a longitudinal centerline axis ofthe gas turbine engine). The variable vane rows have been usedeffectively to schedule the engine around flutter conditions bycontrolling the angle of incidence of the airfoils relative to adirection of flowing airflow. Also, bleed or valve systems are knownwhich bleed airflow downstream from the airfoils to throttle airflow andmitigate flutter. Additionally, airfoil designs are known which tailor aleading edge of each airfoil to obtain improved local airfoil incidenceand adjacent airfoils having different natural frequencies. Finally,having inconsistent airfoil spacing in a forward stage varies theintermittent air pulses communicated to a following airfoil stage, thusreducing natural frequency excitation. Disadvantageously, all of thesemethods result in system compromises, small to moderate performancelosses and may be expensive to incorporate into existing gas turbineengine systems.

Accordingly, it is desirable to provide a gas turbine engine having aclosed-loop flutter sensing system which achieves reduced flutteroperation and minimizes performance losses of the gas turbine engine.

SUMMARY

A gas turbine engine assembly according to an exemplary embodiment ofthe present disclosure includes, among other things, a nacelle, a coreengine casing within the nacelle, a low pressure turbine having apressure ratio that is greater than five, and a bypass passageestablished between the nacelle and the core engine casing. About 80% ormore of airflow entering the engine is moved through the bypass passage.

In a further non-limiting embodiment of the foregoing gas turbine engineembodiment, about 80% of the airflow entering the engine is movedthrough the bypass passage.

In a further non-limiting embodiment of either of the foregoing gasturbine engine embodiments, the gas turbine engine includes a fan and agear train, the gear train reduces the rotational speed of the fanrelative to a shaft of the gas turbine engine. The shaft is rotatablycoupled to a low pressure compressor of the engine.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the gear train is a planetary gear train.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a variable area fan nozzle controls a dischargeairflow area of the bypass passage.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the discharge airflow area extends between thevariable area fan nozzle and the core engine casing.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a controller is operable to move the variable areafan nozzle to change the discharge airflow area associated with thevariable area fan nozzle in response to an airfoil flutter condition.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the controller influences the discharge airflow areaby moving the variable area fan nozzle between a first position having afirst discharge airflow area and a second position having a seconddischarge airflow area greater than the first discharge airflow area inresponse to the airfoil flutter condition.

A gas turbine engine according to another exemplary embodiment of thepresent disclosure includes, among other things, a nacelle, a coreengine casing within the nacelle, a low pressure turbine having apressure ratio that is greater than five, and a bypass passageestablished between the nacelle and the core engine casing. A ratio ofan amount of airflow communicated through the bypass passage to anamount of airflow communicated through the core engine is greater than10.

In a further non-limiting embodiment of the foregoing gas turbine engineembodiment, the gas turbine engine includes a fan and a gear train. Thegear train reduces the rotational speed of the fan relative to a shaftof the gas turbine engine. The shaft is rotatably coupled to a lowpressure compressor of the engine.

In a further non-limiting embodiment of either of the foregoing gasturbine engine embodiments, the gear train is a planetary gear train.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a variable area fan nozzle that controls a dischargeairflow area of the bypass passage.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the discharge airflow area extends between thevariable area fan nozzle and a core engine casing.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a controller is operable to move the variable areafan nozzle to change the discharge airflow area associated with thevariable area fan nozzle in response to the airfoil flutter condition.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the controller influences the discharge airflow areaby moving the variable area fan nozzle between a first position having afirst discharge airflow area and a second position having a seconddischarge airflow area greater than the first discharge airflow area inresponse to detection of the airfoil flutter condition.

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description arebriefly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a general partial cut-away view of a gas turbineengine;

FIG. 2 is a perspective view of a section of a variable area fan nozzle(VAFN);

FIG. 3 is a schematic view of an example gas turbine engine having avariable area fan nozzle (VAFN); and

FIG. 4 illustrates a partial cut-away view of a fan section of the gasturbine engine.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 which suspends from a pylon11 and may include (in serial flow communication) a fan section 12, alow pressure compressor 14, a high pressure compressor 16, a combustor18, a high pressure turbine 20 and a low pressure turbine 22. Duringoperation, air is pulled into the gas turbine engine 10 by the fansection 12, is pressurized by the compressors 14, 16, and is mixed withfuel and burned in the combustor 18. Hot combustion gases generatedwithin the combustor 18 flow through the high and low pressure turbines20, 22, which extract energy from the hot combustion gases.

In a two spool design, the high pressure turbine 20 utilizes theextracted energy from the hot combustion gases to power the highpressure compressor 16 through a high speed shaft 19, and a low pressureturbine 22 utilizes the energy extracted from the hot combustion gasesto power the low pressure compressor 14 and the fan section 12 through alow speed shaft 21. However, the invention is not limited to the twospool gas turbine architecture described and may be used with otherarchitectures such as a single spool axial design, a three spool axialdesign and other architectures. That is, the present invention isapplicable to any gas turbine engine, and to any application.

The example gas turbine engine 10 is in the form of a high bypass ratioturbofan engine mounted within a nacelle 26, in which a significantamount of the air pressurized by the fan section 12 bypasses the coreengine for the generation of propulsion thrust. The nacelle 26 partiallysurrounds a fan casing 28 and an engine casing 31. The exampleillustrated in FIG. 1 depicts a high bypass flow arrangement in whichapproximately 80% of the airflow entering the fan section 12 may bypassthe core engine via a fan bypass passage 30 which extends between thenacelle 26 and the core engine casing 31 for receiving and communicatinga discharge airflow F1. The high bypass flow arrangement provides asignificant amount of thrust for powering an aircraft.

In one example, the bypass ratio (i.e., the ratio between the amount ofairflow communicated through the fan bypass passage 30 relative to theamount of airflow communicated through the core engine itself) isgreater than ten, and the fan section 12 diameter is substantiallylarger than the diameter of the low pressure compressor 14. The lowpressure turbine 22 has a pressure ratio that is greater than five, inone example. The engine 10 may include a gear train 23 which reduces thespeed of the rotating fan section 12. The gear train 23 can be any knowngear system, such as a planetary gear system with orbiting planet gears,a planetary system with non-orbiting planet gears, or other type of gearsystem. In the disclosed example, the gear train 23 has a constant gearratio. It should be understood, however, that the above parameters areonly exemplary of a contemplated geared turbofan engine. That is, theinvention is applicable to a traditional turbofan engine as well asother engine architectures.

The discharge airflow F1 is communicated within the fan bypass passage30 and is discharged from the engine 10 through a variable area fannozzle (VAFN) 40 defined radially between the nacelle 26 and the coreengine casing 31. Core exhaust gases C are discharged from the coreengine through a core exhaust nozzle 32 defined between the core enginecasing 31 and a center plug 34 defined coaxially therein around alongitudinal centerline axis A of the gas turbine engine 10.

In one example, the VAFN 40 concentrically surrounds the core enginecasing 31 near an aftmost segment 29 of the nacelle 26. However, theVAFN 40 may be positioned at other locations of the engine 10. Adischarge airflow area 36 is associated with the VAFN 40 and extendsbetween the VAFN 40 and the core engine casing 31 for axiallydischarging the fan discharge airflow F1.

FIG. 2 illustrates the components of the VAFN 40. This structure isexemplary only, and, as other embodiments would similarly vary thedischarge airflow area 36, will only be briefly discussed herein. TheVAFN 40 generally includes a synchronizing ring 41, a static ring 43 andat least one flap assembly 45. Other VAFN actuation mechanisms may beused. The flap assembly 45 is pivotally mounted to the static ring 43 atmultiple hinges 47 and linked to the synchronizing ring 41 through alinkage 49. An actuator assembly 51 selectively rotates thesynchronizing ring 41 relative to the static ring 43 to adjust the flapassembly 45 through the linkage 49. The radial movement of thesynchronizing ring 41 is converted to tangential movement of the flapassembly 45 to vary the discharge airflow area 36 of the VAFN 40, as isfurther discussed below.

FIG. 3 illustrates a flutter sensing system 50 of the gas turbine engine10. The discharge airflow area 36 may be influenced during certainflight conditions, such as flutter conditions, by opening or closing theVAFN 40. Flutter conditions represent self-induced oscillations. Flutterconditions are caused by unsteady aerodynamic conditions such as theinteraction between adjacent airfoils. During flutter, aerodynamicforces couple with each airfoil's elastic and inertial forces, which mayincrease the kinetic energy of each airfoil and produce negativedamping. The negative damping is enhanced where adjacent airfoils beginto vibrate together.

In one example, the VAFN 40 is moveable between a first position X and asecond position X′ (represented by phantom lines). A discharge airflowarea 37 of the second position X′ is greater than the discharge airflowarea 36 of the first position X.

The VAFN 40 is selectively moved to the second position X′ to controlthe air pressure of the discharge airflow F1 within the fan bypasspassage 30. For example, closing the VAFN 40 (i.e., moving the VAFN tothe first position X) reduces the discharge airflow area which restrictsthe fan airflow F1 and produces a pressure build up (i.e., an increasein air pressure) within the fan bypass passage 30. Opening the VAFN 40to the second position X′ increases the discharge airflow area, allowingadditional fan airflow, which reduces the pressure build up (i.e., adecrease in air pressure) within the fan bypass passage 30. That is,opening the VAFN 40 creates additional thrust power for the gas turbineengine 10.

The flap assemblies 45 (See FIG. 2) of the VAFN 40 are moved from thefirst position X to the second position X′ in response to detecting aflutter condition of the gas turbine engine 10, in one example. Inanother example, the VAFN 40 is moved in response to detecting across-wind condition. However, it should be understood that the VAFN 40may additionally be actuated in response to other operability conditionssuch as take-off or ground operations.

The flutter sensing system 50 is a closed-loop system and includes asensor 52 and a controller 54. The sensor 52 actively and selectivelydetects the flutter condition and communicates with the controller 54 tomove the VAFN 40 between the first condition X and the second positionX′ or any intermediate position via the actuator assemblies 51. Ofcourse, this view is highly schematic. In one example, the sensor 52 isa time of arrival type sensor. A time of arrival sensor times thepassage (or arrival time) of an airfoil as the airfoil passes a fixed,case-mounted sensor as the airfoil rotates about the engine longitudinalcenterline axis A. In the example shown in FIG. 3, the arrival time ofthe fan section 12 airfoils 60 are timed by the sensor 52. Of course,other airfoils may similarly be timed. The controller 54 is programmedto differentiate between which airfoil arrival times correlate to aflutter condition and which airfoil arrival times correlate tonon-flutter conditions.

It should be understood that the sensor 52 and the controller 54 areprogrammable to detect flutter conditions or other conditions. A personof ordinary skill in the art having the benefit of the teachings hereinwould be able to select an appropriate sensor 52 and program thecontroller 54 with the appropriate logic to communicate with the sensor52 and the actuator assembly 51 to move the VAFN 40 between the firstposition X and the second position X′ or any intermediate position inresponse to a flutter condition or any other condition.

The VAFN 40 is returned to the first position X from the second positionX′, which is otherwise indicated when the flutter conditions subside. Inone example, the sensor 52 communicates a signal to the controller 54where the flutter conditions are no longer detected by the sensor 52.Therefore, the efficiency of the gas turbine engine 10 is improvedduring both flutter and non-flutter conditions. Also, airfoil damage dueto continued operation in a flutter condition is reduced.

FIG. 4 illustrates an example mounting location for the sensor 52 of theflutter sensing system 50. In one example, the sensor 52 is mounted tothe fan casing 28 which surrounds the fan section 12. In anotherexample, the sensor 52 is mounted directly adjacent to a blade tip areaT of the fan section 12. The blade tip area T of the fan section 12 isthe area of the fan casing 28 which is directly adjacent to the tips 62of each airfoil 60 (only one shown in FIG. 4) of the fan section 12 asthe airfoils 60 are rotated about the engine centerline axis A. In yetanother example, multiple sensors 52 are circumferentially disposedabout the core engine casing 31 adjacent to the blade tip area T of eachairfoil 60. The sensor 52 may also be mounted adjacent to the blade tiparea of the airfoils of the compressor sections 14, 16 or the turbinesections 20, 22.

The foregoing description shall be interpreted as illustrative and notin any limiting sense. A worker of ordinary skill in the art wouldrecognize that certain modifications would come within the scope of thisinvention. For that reason, the following claims should be studied todetermine the true scope and content of this invention.

What is claimed is:
 1. A gas turbine engine comprising: a fan section including a fan; a fan casing surrounding the fan section; a compressor section defining an engine axis; a gear train that reduces a rotational speed of the fan relative to a shaft; a low pressure turbine coupled to the shaft; a nacelle extending along the engine axis and surrounding the fan casing; a bypass ratio of greater than 10; a variable area fan nozzle defining a discharge airflow area; and a flutter sensing system including a controller programmed to move the variable area fan nozzle to vary the discharge airflow area in response to detecting an airfoil flutter condition associated with adjacent airfoils of the fan.
 2. The gas turbine engine as recited in claim 1, wherein the discharge airflow area extends between the variable area fan nozzle and a core engine casing that surrounds the compressor section.
 3. The gas turbine engine as recited in claim 2, wherein the flutter sensing system includes a sensor in communication with the controller, and the controller is programmed to move the variable area fan nozzle between a first position having a first discharge airflow area and a second position having a second discharge airflow area greater than the first discharge airflow area in response to the airfoil flutter condition.
 4. The gas turbine engine as recited in claim 3, wherein the second position is radially outward from the first position relative to the engine axis.
 5. The gas turbine engine as recited in claim 3, wherein sensor detects the airfoil flutter condition.
 6. The gas turbine engine as recited in claim 5, wherein the sensor is mounted adjacent to a blade tip area defined by the adjacent airfoils.
 7. The gas turbine engine as recited in claim 6, wherein the controller is programmed to move the variable area fan nozzle in response to detecting a cross-wind condition.
 8. The gas turbine engine as recited in claim 6, wherein the sensor is mounted to the fan casing, and the blade tip area is defined as an area of the fan casing directly adjacent to tips of the adjacent airfoils.
 9. The gas turbine engine as recited in claim 6, wherein the low pressure turbine has a pressure ratio that is greater than five.
 10. The gas turbine engine as recited in claim 9, wherein the sensor detects a time of arrival of the adjacent airfoils, and the controller is programmed to differentiate between airfoil arrival times of the adjacent airfoils that correlate to flutter conditions and other airfoil arrival times of the adjacent airfoils that correlate to non-flutter conditions.
 11. The gas turbine engine as recited in claim 10, wherein the controller is programmed to move the variable area fan nozzle in response to detecting a cross-wind condition.
 12. The gas turbine engine as recited in claim 5, wherein the sensor detects a time of arrival of the adjacent airfoils, and the controller is programmed to differentiate between airfoil arrival times of the adjacent airfoils that correlate to flutter conditions and other airfoil arrival times of the adjacent airfoils that correlate to non-flutter conditions.
 13. The gas turbine engine as recited in claim 12, wherein the sensor is a plurality of sensors circumferentially disposed about the core engine casing.
 14. The gas turbine engine as recited in claim 12, wherein the second position is radially outward from the first position relative to the engine axis.
 15. The gas turbine engine as recited in claim 14, wherein the variable area fan nozzle includes a synchronizing ring, a static ring and at least one flap assembly, the at least one flap assembly pivotally mounted to the static ring and linked to the synchronizing ring.
 16. The gas turbine engine as recited in claim 15, wherein the engine is a two spool engine comprising a low spool and a high spool, the compressor section including a low pressure compressor and a high pressure compressor, the low spool including the low pressure compressor driven by the low pressure turbine, the low pressure compressor including a plurality of stages, the low pressure turbine being a three-stage turbine, the high spool including a two-stage high pressure turbine that drives the high pressure compressor, and the gear train has a constant gear ratio.
 17. A gas turbine engine comprising: a fan section including a fan; a fan casing surrounding the fan section; a compressor section and a turbine section extending along an engine axis, the turbine section including a high pressure turbine and a low pressure turbine that drives a shaft, the low pressure turbine having a pressure ratio that is greater than five; a gear train interconnecting the fan and the shaft; a nacelle surrounding the fan casing; a variable area fan nozzle axially aft of the fan casing relative to the engine axis, the variable area fan nozzle moveable relative to the engine axis to influence a discharge airflow area associated with the variable area fan nozzle; an actuator assembly that moves the variable area fan nozzle between a first position and a second position relative to the engine axis; and a flutter sensing system including a sensor and a controller programmed to communicate with the actuator assembly to move the variable area fan nozzle to vary the discharge airflow area in response to the sensor detecting an airfoil flutter condition.
 18. The gas turbine engine assembly of claim 17, wherein the shaft is rotatably coupled to a low pressure compressor having a plurality of stages, and the low pressure turbine is a three-stage turbine.
 19. The gas turbine engine as recited in claim 18, further comprising a bypass ratio of greater than 10, and wherein the second position is radially outward from the first position relative to the engine axis.
 20. The gas turbine engine as recited in claim 17, wherein the sensor is a plurality of sensors circumferentially disposed about a core engine casing, each of the plurality of sensors being a time of arrival type sensor.
 21. The gas turbine engine as recited in claim 17, further comprising a bypass ratio of greater than 10, and wherein the first position defines a first discharge airflow area and the second position defines a second discharge airflow area greater than the first discharge airflow area.
 22. The gas turbine engine as recited in claim 17, wherein the sensor is mounted adjacent to a blade tip area defined by adjacent airfoils.
 23. The gas turbine engine as recited in claim 22, wherein the sensor is coaxially aligned with the blade tip area with respect to the engine axis.
 24. The gas turbine engine assembly of claim 22, wherein the discharge airflow area extends between the variable area fan nozzle and a core engine casing.
 25. The gas turbine engine as recited in claim 24, further comprising a bypass ratio of greater than 10, and wherein the sensor is mounted adjacent to a blade tip area defined between the fan casing and the adjacent airfoils.
 26. The gas turbine engine as recited in claim 25, wherein the second position is radially outward from the first position with respect to the engine axis, the actuator assembly includes a synchronizing ring, a static ring and a plurality of flap assemblies extending about the engine axis, the plurality of flap assemblies pivotally mounted to the static ring and linked to the synchronizing ring, and the controller is programmed to communicate with the actuator assembly to rotate the synchronization ring relative to the static ring to adjust the plurality of flap assemblies and vary the discharge airflow area in response to the sensor detecting the airfoil flutter condition.
 27. The gas turbine engine as recited in claim 26, wherein the controller is programmed to cause the actuator assembly to move the variable area fan nozzle in response to detecting a cross-wind condition.
 28. The gas turbine engine as recited in claim 25, wherein the sensor detects a time of arrival of the adjacent airfoils, and the controller is programmed to differentiate between airfoil arrival times of the adjacent airfoils that correlate to flutter conditions and other airfoil arrival times of the adjacent airfoils that correlate to non-flutter conditions.
 29. The gas turbine engine as recited in claim 28, wherein the high pressure turbine is a two-stage turbine.
 30. The gas turbine engine as recited in claim 22, wherein the adjacent airfoils are located in the compressor section or the turbine section. 